A pulse amplitude modulation 4-level PAM4 SerDes communications link is based on PAM4 signal encoding with four signal levels that encode 2 bits mapped to one symbol, such as based on Gray coding: 10/11/01/00, or 3201. The PAM4 signal levels are separated by three PAM4 eyes: top, middle, and bottom.
The three PAM4 eyes are not vertically symmetrical (eye height and width) because, with four signal/voltage levels, transitions are required between non-adjacent signal levels, which take longer than transitions between adjacent signal levels. As a result, the top and an bottom eyes are narrowed relative to the middle eye.
Signal path nonlinearities cause significant degradation in the PAM4 signal, as reflected in eye symmetry (eye height/width) and vertical alignment (skew). For an example system with a transmitter TX and receiver RX connected over a PAM4 SerDes com link, signal path nonlinearities in the TX, RX and com link (including TX-end and RX-end signal interfaces) degrade the PAM4 signal (see, FIG. 3A), as reflected in degrading vertical eye symmetry and vertical eye alignment.